The Effects of Soil Moisture Content on Reflectance Spectra of Soils Using UV-VIS-NIR Spectroscopy

This study was conducted to investigate the effects of soil moisture content on reflectance spectra of sandy soils with different phosphorus (P) concentrations using UV-VIS-NIR reflectance spectroscopy. Sandy soils with average particle size of 250 μm were used. Investigations were conducted at 0, 12.5, 62.5, 175, 375, 750, and 1000 mg/kg phosphorus application rates. Three soil moisture contents, i.e., 4%, 8%, and 12 % were investigated. P concentrations of the soil samples were analyzed and reflectance of the samples was measured between 175-2550 nm with a 1 nm interval. Dried soil sample reflected more lights than wet soils in 225-2550 nm. As moisture content of soils increased, reflectance from the soil sample decreased, which indicates that water is a strong light absorber than a sandy soil sample. Wet soil spectra was reconstructed by removing the moisture content effect and compared with the dry spectra of the same soil sample. Absorbance and reconstructed absorbance data were prepared as calibration and validation data sets in order to measure the performance of the spectral signal processing used in the removal of the effect of moisture content on absorbance spectra. A partial least squares (PLS) analysis was applied to the data to predict P concentration before and after processing the spectra. The results showed that spectral signal processing by removing the moisture effect improved considerably P prediction in soils.